Much effort is expended in attempts to effectively secure premises from intruders. Human guards provide the most versatile but not necessarily the most reliable or cost-effective protection. Guards can patrol for only a limited number of hours at a time, are sometimes drowsy and inattentive during patrol, and are expensive.
There is a strong need for automated security systems which supplement or replace patrolling guards. Perimeter alarm systems such as door and window sensors attempt to establish a barrier about the exterior of the premises at possible points of entry. Precisely because they are located at these known points they can be circumvented by knowledgeable intruders.
Automated protection within a premises is typically provided by one or more types of fixed volumetric detectors also known as space alarms. Stationary microwave devices emit microwave radiation at a known frequency and analyze returned radiation to determine changes in frequency induced by moving objects. Motion of an intruder produces a Doppler-effect shift in the received radiation. Passive infrared detectors provide another type of volumetric detection. These detectors receive infrared radiation to determine when a heat source such as a human body moves within its field of view. Ultrasonic detectors emit high-frequency sound waves which are reflected by objects in its path. A change in the amount or frequency of returned radiation indicates the approach of an intruder.
Each of the above-mentioned types of volumetric detectors senses a particular condition within its field of view. Accordingly, each is susceptible to particular non-intruder events which generate a false alarm. Changes in building climate control, start-up of ventilation fans and turbines, and other non-threatening events may interfere with the sensors. While increasing sensitivity of the detectors increases the likelihood of intruder detection, it also increases the chances of costly false alarms. In some areas it is necessary to decrease sensitivity to avoid false readings. Further, the reliability of individual sensors poses problems since they eventually fail over time.
In an attempt to improve reliability without sacrificing sensitivity, some systems combine two or more types of sensors and align them to share fixed, overlapping fields of view. A simple AND circuit or comparator approach requires simultaneous changes in the two or more sensed conditions before generating an alarm.